This invention relates to a resin composition for information signal recording discs (hereinafter referred to simply as "information discs" or "discs") comprising nitrile copolymers, which has excellent transparency, molding fidelity and anti-electrification.
A variety of video discs, audio discs and the like have been known wherein information signal pits are engraved on one side of the disc, and a metal coating is deposited thereon, the information signals recorded onto the pits being reproducible by irradiation thereof with laser rays from the other side of the disc.
As materials for fabrication of such discs, rigid vinyl chloride resins have been mainly considered. Some problems, however, have been pointed out with respect to the rigid vinyl chloride resins used for this purpose. The first problem is that various additives such as thermal stabilizers, antistatic agents, lubricants and releasing agents are admixed into the vinyl chloride resins, whereby these additives bleed out of the surface of the resulting disc with time, leading to deterioration of the transmittance of light rays. On the other hand, when the amounts of these additives are decreased to suppress the bleeding, continuous molding of the discs becomes difficult. The second problem relating to these rigid vinyl chloride resins is that there has been no satisfactory antistatic agent which exhibits antistatic effect without impairing thermal stability and bleed resistance. As a result, even a slight friction gives rise to attraction of dust to the discs, which becomes a cause of noise. A variety of apparatuses have been designed to solve such problems but are still far from being fully satisfactory.
The present inventors have been engaged in research for many years on novel resins which can be effectively used in place of such rigid vinyl chloride resins and have found that a specific nitrile resin has a good antistatic property and excellent thermal stability and, moreover, requires no auxiliaries such as antistatic agents, lubricants, releasing agents, and thermal stabilizers. Thus, these nitrile resins can maintain transparency and antistatic properties for a very long time in comparison with the conventional vinyl chloride resins (as set forth in Japanese patent applications Nos. 68406/76, 72022/76 and 148538/76, and U.S. Pat. No. 4,125,654).
The present inventors have carried out further research on the optimum materials for information discs from which recorded information is reproducible by irradiation with laser rays. The resin material for such information discs especially requires, in addition to the above-mentioned anti-electrification, transparency to amply transmit laser lays, and molding fidelity so that the minute concavities and convexities of a stamper for engraving information pits of high recording density can be produced on the information discs with fidelity. It is known that such molding fidelity is not only related to the melt viscosity at the molding temperature of the resin material but also is closely related to the melt elasticity of the resin material.
The above-mentioned nitrile resins which the present inventors have already developed (e.g., as disclosed in Japanese Patent Application 68406/76, U.S. Pat. No. 4,125,654, etc.) do not fully satisfy these properties, especially transparency.